Code Division Multiplexing (CDM) techniques are widely applied in the technical field of wireless communication. The most classical CDM technique is to expand different signals by using different orthogonal sequences, and superpose them so as to eliminate interferences between the superposed signals by means of orthogonal property among the different sequences. Because of this advantage, CDM techniques are widely applied for multiplexing different signals in a communication system.
FIG. 1(A) to FIG. 1(D) are diagrams showing the principle of the CDM multiplexing using four-dimension Walsh codes. As shown in FIG. 1(A), the code words used in CDM are orthogonal to each other, which means the correlations among different code words are zero. As shown in FIG. 1(B), in CDM multiplexing, different signals S1, S2, S3, S4 correspond to the different code words respectively, and those different signals are multiplied by the corresponding code words respectively. The results of the multiplication produce expansions of signals. Expansions produced by the different signals are superposed to form multiplexed signals W, X, Y, Z. As shown in FIG. 1(C), the multiplexed signals W, X, Y, Z are transmitted on communication channels. Expansions of signals by CDM may be performed either on time domain or frequency domain. As shown in FIG. 1(D), in CDM de-multiplexing, the signals are correlated with the different code words to recover the original signals S1, S2, S3, S4.
In the CDM multiplexing using orthogonal codes, the orthogonality among different orthogonal code words is the most essential characteristic. In wireless communication, the most widely used orthogonal code is Walsh code, but the length of such code can only be 2, 4, 8, 16 . . . (power of 2). For the orthogonal sequences with other lengths, other construction methods are used. For example, an orthogonal sequence with a length of 3 may be obtained by a 3*3 Fourier Transform matrix.
In CDM multiplexing, signals before multiplexing are expanded by the CDM code words in the procedure of multiplexing. In a communication system, such expansion may be either on the time domain or on the frequency domain. The orthogonality requires the channels where the expanded signals are superposed to be flat (invariable). However, due to the selectivity of time and frequency of the communication channels, such a flat property is not always obtained. If the selectivity exists in the channels after expansion, in the CDM de-multiplexing, the orthogonality between the different code words is deteriorated. The stronger the selectivity is, the severer such deterioration is. Further, due to characteristics of the orthogonal codes, the CDM de-multiplexing has to correlate all signals after expansion, thereby increasing the difficulty of the CDM de-multiplexing.